refactor(slint): one shared TrackRow + TrackItem across all surfaces

Collapses the three near-identical track rows (AlbumView's inline
TrackRow, ArtistPage's inline TrackRow with multi-select, and the
shared SearchTrackRow) into a single primitives/TrackRow driven by
flags, backed by one TrackItem (superset). Foundation for the
upcoming playlist module, which renders the artwork variant heavily.

  - New primitives/TrackRow.slint with: show-artwork (cover+number vs
    number cell), show-album, show-favorite, show-download,
    multi-select-mode (selection checkbox). One media-action wires
    play / play-next / queue / favorite / download / toggle-select +
    the TrackContextMenu.
  - TrackItem is the unified model (superset of the old AlbumTrackItem
    + SearchTrackItem: number, album, selected, artwork all present;
    unused fields stay empty per surface).
  - Migrated AlbumView (show-artwork:false, show-download:true),
    ArtistPage Popular (show-artwork:false, multi-select-mode),
    Search, Favorites, Mix (show-artwork:true, +album/+favorite).
    Deleted SearchTrackRow.slint and both inline TrackRows.
  - Rust: AlbumTrackItem + SearchTrackItem -> TrackItem across
    album/artist/search/favorites/mix/main; search's internal row
    struct renamed TrackRow -> TrackRowData to free the TrackRow name
    for the Slint component. 46 tests pass.

cc

crates/qbz-slint/ui/favorites/FavoritesView.slint

@@ -1,7 +1,7 @@
 // Library > Favorites — the user's saved tracks / albums / artists /
 // playlists / labels. Mirrors Tauri's FavoritesView.svelte core: a
 // tab bar over per-tab content. Tracks reuse the shared
-// SearchTrackRow, albums the Discover AlbumCard, artists a round-
+// TrackRow, albums the Discover AlbumCard, artists a round-
 // portrait grid. Playlists + Labels are stubbed pending their
 // backing views.
 
@@ -12,13 +12,13 @@ import {
     FavoritesState,
     FavoritesActions,
     AlbumCardItem,
-    SearchTrackItem,
+    TrackItem,
     FavoriteArtistItem,
     FavoritePlaylistItem,
     FavoriteLabelItem,
 } from "../state.slint";
 import { AlbumCard } from "../discover/AlbumCard.slint";
-import { SearchTrackRow } from "../search/SearchTrackRow.slint";
+import { TrackRow } from "../primitives/TrackRow.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { LoadingSpinner } from "../primitives/LoadingSpinner.slint";
 
@@ -323,7 +323,7 @@ export component FavoritesView inherits Rectangle {
                     color: Theme.text-muted;
                     font-size: Typography.body;
                 }
-                for track[i] in FavoritesState.tracks: SearchTrackRow {
+                for track[i] in FavoritesState.tracks: TrackRow {
                     track: track;
                     index: i;
                     show-artwork: true;